This invention relates to alpha-numeric character display by electrical means (e.g. on the screen of a television monitor or using an electrically-operated printer device), and more especially to character generator arrangements for controlling the display of characters in a character display system.
It is known to use in such a character generator arrangement a "read-only" memory device, which serves as a character store in which characters are stored in a manner representing a character format composed of a co-ordinate matrix of discrete character elements, and which is responsive to character input data, representing alpha-numeric characters, to produce character generating data which can be used for the display of the alpha-numeric characters.
Such a character generator arrangement may therefore be of a kind comprising, in addition to the character store, input addressing means for addressing the character store with character input data for a character selected for display, row selection means for producing row addresses for selecting, for a read-out operation from the character store, those character elements in a particular character element row of the character matrix, data output means for producing in said read-out operation the character generating data for the character elements in the selected character element row of the selected character, and logic control means for so controlling the input addressing means and the row selection means as to cause, for a plurality of characters to be displayed as a row of characters, the selection in turn of the character elements in a first character element row of each of said plurality of characters, then the selection in turn of the character elements in a second character element row thereof, and so on for their remaining character element rows, so that said plurality of characters are built-up element row-by-element row, as a whole, for display.
In order to limit the size of a "read-only" memory device which serves as a character store, it is known to provide for a particular character format a co-ordinate character matrix of n character element rows which is just large enough to accommodate each of the upper case letters of the alphabet. The same size of character matrix can also accommodate each of the corresponding lower case letters. However, the lower case letters g, j, p, q, and y, which are known as descender letters because they have "tails" which should descend below the bottom of a displayed character row in which they occur, can only be accommodated in their respective character matrix if they are, in effect, stored in a "raised" position in the matrix, since otherwise their "tails" would lie outside the matrix and thus would not be accommodated therein. As a consequence, a character generator arrangement of the above kind having such a character store requires additional circuitry in its logic control means for the read-out of these descender letters in order to "lower" a descender letter, as displayed, into its correct position in a character row. Without such additional circuitry a displayed descender letter would be out of alignment in the character row.
It is known for the additional circuitry to include an adder circuit to which the row addresses from the row selection means are applied and which is made operable by detector means when said character input data represents a descender letter to add to each successive row address applied to it a number appropriate for changing the row address to delay the read-out operation, in respect of the descender letter, by a number of character element rows corresponding to said number, whereby the descender letter as displayed is lowered in position by such number of character element rows so as to be in correct alignment in a displayed character row.
The lowering of the descender letters in this way requires the row selection means to provide a number of additional row addresses, and the character generator arrangement to further include enabling means responsive to said detector means for enabling said data output means to produce character generating data for a first n successive row addresses for normal read-out operation and for a second different, n successive row addresses for descender letter read-out operation. Mullard Technical Note 7 (TP1404), October 1974, describes a character generator arrangement in which the character store is a ROM (Read Only Memory) for which, essentially, the ASCII code is used for the read out of characters stored on a 7.times.5 dot matrix basis, and in which arrangement an adder circuit is used, as aforesaid, to effect inter alia the correct alignment of descender letters in a row of characters.
However, this additional circuitry which is required for the correct alignment of the descender letters has the disadvantage that because the adder circuit has to operate at the read out rate for the character generating data, which will usually be a fast rate, of for example 1 MHz, this adder circuit will be complex in implementation.